DENVER — When it was going well — and it was really going well for a spell — Tim Tebow’s popularity was so polarizing that, had it continued, he probably could have wrapped up the Republican Presidential nomination by now and these contests in Iowa and New Hampshire would be a waste of time.

But the Tebow train has been off the rails of late, and that widespread popularity he built has morphed into curiosity and doubt. Not long ago the darling of the league, Tebow now faces more scorn and criticism than President Obama at a Republican mixer.

The Tebow story will be played out at 4:30 p.m. today when the 8-8 Broncos play the heavily favored 12-4 Steelers in an AFC wild-card game at Mile High.

Many expect the Steelers, with their No. 1-ranked defense and postseason-tested starting quarterback Ben Roethlisberger, to annihilate the reeling Broncos, who have lost their past three games while their offense has disappeared into the thin Rocky Mountain air.

Tebow’s production has regressed in recent weeks, leading to backup Brady Quinn taking added snaps in practice, a sure sign that the leash on Tebow might be short today.

Tebow’s completion percentages in his last five games: 66.7, 52.5, 50.0, 44.8, 27.3. For the season, he finished last among NFL starting quarterbacks in completion percentage at 46.5 percent.

During the three-game losing streak the Broncos bring into today’s game Tebow completed 30 of 73 passes (41.1 percent) with only one TD pass, four INTs, three lost fumbles and was sacked 10 times.

Even his rushing production has been stifled. After averaging 63 yards per game his first nine starts, Tebow has been held to 25 per game the past two as defenses have hemmed him in the pocket, forcing him into a discomfort zone.

“He’s not an anticipator of throws,’’ said CBS analyst Phil Simms, who will call today’s game. “That’s why he likes to move and throw the ball. He wants to scramble because then it becomes, ‘Oh, there he is, I’ll throw it to him.’

“Think about his career. In high school, if he didn’t like it, he ran. In college, if he didn’t like it he ran. In the pros, if he doesn’t like it, he runs. There’s a limit to that. If he wants to be successful, he’s got to let [the ball] go. He’s got to have faith in his left arm.’’

Teams are figuring Tebow out, and now it’s on him to get better and make teams pay for stacking eight and nine men in the box daring him to beat them over the top with the pass.

“I think that he’s probably going through what a lot of new starting quarterbacks go through in that you get enough looks of people on film, then you get ideas about[his] comfort zone and how you can play him,’’ Steelers coach Mike Tomlin said.

Broncos executive vice president of football operations, John Elway, weighed in this week with some pointed advice for Tebow, calling for him to “pull the trigger’’ and stop being so tentative.

“That’s human nature, especially when you’re young, to become cautious,’’ Elway said in an interview with the Denver Post. “The key thing for him is to go out, put everything behind him, go though his progressions and pull the trigger.

“When you get into playoff situations, he’s a good enough athlete, you know, to pull the trigger. I have full confidence he’ll bounce back and have a good week.’’

If he doesn’t, the Broncos will be toast against the Steelers and the voices of the Tebow doubters will grow louder over the offseason.

Tebow took a 1-4 team and led it to seven wins in eight games, including six in a row, to make the Broncos relevant and help lead them to the playoffs for the first time since 2005. But those accomplishments quickly are fading into the background with each struggling performance.